The present invention pertains to apparatus for the continuous casting of metals, particularly aluminum and its alloys, and the invention is more particularly concerned with a new and improved form of roll caster and method of operating the same.
The roll casting machine is characterized by a pair of parellel casting rolls which are spaced apart slightly to receive molten metal between them, a pouring tip fitted snugly into the converging space between said casting rolls on the entrance side thereof, and means for driving said rolls. The rolls are usually water-cooled to chill the molten metal and solidify the same. A good example of the prior roll caster described above is shown and described in U.S. Pat. No. 2,790,216, which issued Apr. 30, 1957, to J. L. Hunter.
The Hunter continuous casting machine is well known in the industry, and has enjoyed a large measure of commercial success because it produces a high quality of aluminum strip at a fairly good rate of production. The commercially available Hunter caster has 24 -inch diameter rolls and produces 0.250 inch thick strip of the softer aluminum alloys (e.g. alloy No. 1100, for example) at the rate of 40 to 45 inches per minute. In the Hunter caster, complete solidification of the molten metal takes place slightly ahead of the centerline of the rolls, and this solidified metal is then reduced in thickness by some 15 to 20% as the metal advances through the diminishing space between the rolls, until it passes through the roll centerline, where the roll spacing is at the minimum. Thus, the Hunter caster provides simultaneous casting, solidification, and a slight amount of hot rolling, which produces a crystal grain structure that is essentially "as cast" structure, except that the dendrites have been laid down somewhat, and are oriented at an acute angle to the surface, due to the rolling action.
This typical orientation of the crystal structure gave the metal produced by the Hunter caster certain advantages over that produced by other continuous strip casting machines, such as "band casters", but the metal still suffered from many of the handicaps inherent in the "as cast" structure, particularly where subsequent cold work was relatively slight. For example, deep drawing of heavy gauge metal frequently results in severe "earing" of the metal. However, for any application where cold work was sufficient, as in rolling foil, the traditional Hunter cast metal was of excellent quality, and its relatively large, dendrite crystal structure was no handicap.
Before going on to the present invention, it might be well to digress for a moment to discuss what happens to any crystalline metal structure (particularly aluminum and its alloys) during casting, hot working, cold working, and annealing. In conventional casting processes, molten metal is usually poured into or through a mold. Cooling of the molten metal and subsequent solidification is obtained primarily through the mold walls and later, by cooling the metal walls, as with water sprays or air blasts. The resulting "as cast" crystalline structure comprises a relatively thin skin of small-grain structure along the outer surface due to the violent "chill" of the mold; the said skin surrounding the main body of large, needle-shaped dendrite crystals forming the body of the casting; and there being a central inner area where the dendrites growing perpendicular to the mold surface meet. This central inner area is usually an area of heavy segregation of impurities. The grain structure obtained on a "band caster" (e.g., the Hazelett caster) is very similar to the grain structure described above, since the heat transfer and metal solidification follow the same general pattern.
The particular grain structure described above (usually referred to as "as cast" structure), is not suitable for most applications, and to obtain a grain structure suitable for commercial application, the "as cast" structure must be completely destroyed and regenerated through a cycle of deformation (hot or cold rolling), and heat treatment, which produces a phenomenon known as "recrystallization".
When a crystalline metal structure is subjected to sufficient internal stress, the original crystalline structure is fractured. If the material is heated (either instantaneously with the internal stressing or at a later time) to the recrystallization temperature (which, in the case of aluminum alloys will usually be in the range of 650.degree. to 750.degree. F), "centers of recrystallization" are formed along the fractured grain boundaries. The higher the internal stresses, the more centers of recrystallizaion are formed, and the finer the ultimate grain size. The higher the temperature to which the stressed metal is exposed, the quicker the recrystallization takes place. There is also a relationship between stresses required at different temperatures to trigger the recrystallization phenomenon, as heat increases the molecular and crystalline mobility. The finest grain size is achieved with heaviest internal stresses (to produce the largest number of centers of recrystallization) and heating the metal to an elevated temperature just sufficient to give enough time for the newly formed grains to "take over" the full metal volume. If the metal is exposed to the high temperature beyond the optimum time interval, there is a tendency of the larger grains to absorb the smaller grains, with the result that the grain structure becomes larger and coarser.
Recrystallization is customarily achieved by either of two processes: (1) cold rolling, followed by heat treatment; or (2) hot rolling.
In the cold rolling process, hot rolled sheet, with its given grain structure, is cold rolled at varying degrees, usually 35 to 90% total reduction, depending on the metal alloy and the product. The hot rolled grain structure is crushed, and heavy internal stresses are imparted to the metal, but no recrystallization take place (under normal circumstances) because the temperature during the cold rolling cycle is too low, and the metal is in a "frozen" state. The metal is then heat-treated, or annealed, by raising the temperature to a sufficiently high level to cause centers of recrystallization to form. New grains then start to grow around these centers, and if the exposure to high temperature is sufficiently long, the new grain will completely replace the old grain, and the metal will be completely recrystallized.
Hot rolling is usually done to transform cast metal ingots, or slabs, into a thinner sheet product, which may be the finished product, or it may be cold-rolled to finish gauge. The chief benefit of hot rolling is that there is a considerable economy due to energy savings and to reduction of equipment size. If hot rolling is performed at sufficiently high temperatures, and if the reduction ("draft") of a particular rolling pass is sufficient to impart to the metal sufficient internal stresses, then a recrystallization cycle is triggered during and immediately after the rolling cycle.
The original grain structure has a great deal of influence on the final structure, and to eliminate all of the adverse effects from the "as cast" structure (low ductility, elongation, drawability, etc.), the metal must go through an extremely heavy cycle of hot and/or cold work, and repeated recrystallization cycles, until the metal has been completely recrystallized down to the finest possible grain size.
The conventional Hunter casting machines, and all other casting machines known to me at this time, produce what is basically an "as cast" structure, with all of the disadvantages and adverse physical characteristics of "as cast" metal. Metal sheet or strip produced by these machines must be completely recrystallized by a combination of hot and/or cold rolling, together with heat treatment, all of which require expensive equipment, consumption of large amounts of energy, and high labor cost.